Problem with loadMask in Ext.window.Windiw

In my real project I use default loadMask for Ext.grid.Panel om my Ext.window.Window. Mask from Ext.grid.Panel overlaps Ext.window.Window. What should I do to avoid this problem or this is Ext bug? This small example to reproduce this problem.

I see, the width of the grid is wider than the window, the scroll bars are for the window not the grid. Why not give the window layout : 'fit' and remove the width and height from the grid? If you want the grid to scroll horizontally then give the columns width that will make it scroll.

This is simple example where I want to show You problem with loadmask. In my real project I have many panels in application, then i bring Ext.window.Window where I have several grids(some of grids is not visible, I need to scroll for see them), and loadmask for those grids appears outside window, because loadmask is floating component. If component is not visible loadmask should not appear.